Distractor instrument

ABSTRACT

A distractor instrument for use in knee joint surgery is provided that includes a tibial plate, a femoral plate, a tibial arm having a plate end and a control end, wherein the tibial plate is fastened to the plate end of the tibial arm at its anterior edge, and wherein, when the tibial plate is engaged with the tibia, the tibial arm is configured to extend from the tibial plate generally medially or laterally of the knee joint. The distractor instrument also includes a femoral arm having a plate end and a control end, wherein the femoral plate is fastened to the plate end of the femoral arm, and wherein, when the femoral plate is engaged with the femur, the femoral arm extends from the femoral plate generally aligned with the tibial arm. The tibial arm and the femoral arm are pivotably connected to one another at a point between their respective plate and control ends so that pivotal movement of the femoral arm relative to the tibial arm reduces the distance between their control ends and causes the distance between the tibial plate end and the femoral plate end to increase.

This invention relates to a distractor instrument for use in a kneejoint surgery.

It is important to locate components of a knee joint prosthesisaccurately relative to the patient's tibia and femur in order for theimplanted joint prosthesis to function reliably. Accurate location ofthe prosthesis components relative to the bone depends on accuratepreparation of the tibia and femur, in particular that the bones areresected accurately.

It is common for the tibia to be resected first. Techniques for locatingthe tibial resection plane are well known; a suitable technique mightinvolve for example use of an extra-medullary alignment rod.

Preparation of a femur for implantation of the femoral component of aknee joint prosthesis generally involves performing distal, anterior,posterior, anterior chamfer and posterior chamfer cuts. Factors whichshould be taken into account when determining the location of these cutsinclude the spacing between the femur and tibia that is required toachieve appropriate tension in the joint soft tissue, with the joint inboth flexion and extension. The first cut that is performed on the femuris generally the distal cut or the anterior cut. It can be desirable todetermine the plane or the orientation or both of these cuts withreference to the resected tibia.

Location of the orientation or the plane of a femoral cut with referenceto the resected tibia requires that the medial and lateral ligamentswhich span the joint are placed under tension. Devices for tensioningthe ligaments are known, for example as disclosed in EP-A-979636 andU.S. Pat. No. 4,566,448. U.S. Pat. No. 5,431,653 and U.S. Pat. No.5,649,929 relate to ligament tensioning devices which comprise twopivotally interconnected arms. Each of the arms has a plate at one endfor engaging the articulating surfaces of the femur and tibiarespectively. The plates on the arms can be forced apart to applytension to across the joint by relative pivotal movement of the arms. Alocking device between the arms can lock the plates in a spaced apartposition to maintain the joint ligaments under tension.

The arms used in the devices disclosed in U.S. Pat. No. 5,431,653 andU.S. Pat. No. 5,649,929 enable tension to be across the joint but theinstrument has to be removed from the joint before subsequent steps inthe procedure can be performed because it obstructs access to the jointspace.

The present invention provides an instrument assembly for use in kneejoint replacement surgery, comprising pivotally connected arms havingoppositely arranged plates at one end, which are arranged to extendgenerally medially or laterally of the knee joint.

Accordingly, in one aspect, the invention provides a distractorinstrument for use in knee joint surgery, which comprises:

a tibial plate;

a femoral plate;

a tibial arm having a plate end and a control end, wherein the tibialplate is fastened to the plate end of the tibial arm at its anterioredge, and wherein, when the tibial plate is engaged with the tibia, thetibial arm is configured to extend from the tibial plate generallymedially or laterally of the knee joint;a femoral arm having a plate end and a control end, wherein the femoralplate is fastened to the plate end of the femoral arm, and wherein, whenthe femoral plate is engaged with the femur, the femoral arm extendsfrom the femoral plate generally aligned with the tibial arm; andwherein the tibial arm and the femoral arm are pivotably connected toone another at a point between their respective plate and control endsso that pivotal movement of the femoral arm relative to the tibial armto reduce the distance between their control ends causes the distancebetween the tibial plate end and the femoral plate end to increase.

The instrument of the invention has the advantage that it allowsprocedures to be performed on the knee joint while tension is appliedacross the joint. This is made possible by the fact that the arms bywhich tension is applied across the joint extend generally in themedial-lateral direction. The extension of the arms generally in themedial-lateral direction can mean that obstruction of access to thejoint space anteriorly of the joint by the arms is minimised.Furthermore, the size of the incision through which the plates of thedistractor instrument are inserted into the joint space can be madesmaller than is required for certain other joint tensioning instruments.Significantly, the extension of the arms generally in the medial-lateraldirection means that tension can be applied across the joint using thedistractor instrument while the patella lies in or close to its anatomicposition. In particular, it will often not be necessary to displace(sublux) the patella significantly or to evert it in order to fit theplates into the space between the tibia and the femur. This provides theadvantage that the procedure can be performed through a smaller incisionthat might be necessary if the patella is to be subluxed or everted. Italso means that forces that are applied across the joint by ligamentsthat are connected to the patella do not include non-anatomic medial andlateral components. Accordingly, forces applied by patella ligaments donot affect tension in the medial and lateral ligaments.

It will be appreciated that the advantages referred to above can beachieved obtained when the medial and lateral arms do not extenddirectly along or parallel to the medial-lateral axis. The portion ofthe arms closest to the joint will generally extend close to themedial-lateral axis. However, it might be that the angle between themedial-lateral axis and one or each of the arms is greater than 0°, inthe anterior-posterior direction or in the superior-inferior directionor both. The advantages can be obtained when the angle between the axisand at least a portion of one or each of the arms is 10° or more, forexample at least about 20°, or at least about 30°.

The use of the plate and arm assembly of the instrument can also allow apoint of reference for subsequent procedural steps to be establishedquickly and easily compared with other techniques which might requiresteps such as drilling into bone and fixing reference instruments bymeans of pins or screws or both.

The distractor instrument of the invention can be used to mount anancillary instrument. The ancillary instrument will generally be onewhose position relative to the tibial resection plane should becontrolled. For example the instrument might be used to measure thedistance from the tibial resection plane to the femur to enable thecorrect selection of joint prosthesis components to ensure appropriateligament tension. Such an instrument might include a probe forcontacting the femur to measure the gap between the tibia and the femur.

The ancillary instrument might have a slot formed in it for receivingthe blade of a saw or other cutting device. Such an instrument can beused to define the plane for resecting the femur.

The ancillary instrument might be a pin guide which has at least onehole extending through it to define the location for a hole which is tobe prepared in the femur. The pin guide might have a slot formed in itfor receiving the blade of a saw or other cutting device. Holes whichare located using the pin guide might be used for fastening anothercutting guide instrument (with a slot for receiving the blade of a sawor other cutting device) to the femur.

Preferably, the ancillary and distractor instruments have complimentaryformations of a spigot and socket assembly which can be used to connectthe instruments to one another. Preferably, the spigot is a snug fit inthe socket so that play between the distractor and ancillary instrumentsis minimised. It can be preferred for the spigot to be in the form of aplate and the socket to be in the form of a slot in which the plate canbe received. Preferably, the width of the slot is at least about 20 mm,more preferably at least about 25 mm, for example at least about 30 mm.Preferably, the depth of the slot is at least about 8 mm, morepreferably at least about 10 mm, for example about 12 mm. Preferably,the height of the slot is at least about 1.5 mm, more preferably atleast about 2.0 mm, for example about 2.5 mm.

The depth of the plate which fits into the slot can be greater than thedepth of the slot when the slot is open at its opposite end, so that theplate protrudes from the slot when fully inserted.

The thickness of the plate should be only slightly less than the heightof the slot, so that it is a tight sliding fit in the slot, allowing theplate to be slid smoothly into the slot but so that play between theplate and the slot is minimised.

It can be preferred for the width of the plate to be less than the widthof the slot to allow the position of the plate relative to the slot,along the axis of the arm, to be changed slightly by sliding the platein the slot. This can enable the position of an ancillary instrumentrelative to the knee joint to be adjusted generally in the mediallateral direction.

It will often be preferred that the spigot and socket of the assemblylie on an axis which is approximately parallel to the plane that is tothe face of the tibial plate which contacts the resected tibia.

It will generally be preferred for the socket to be provided on thedistractor instrument and the spigot to be provided on the ancillaryinstrument. This has the advantage that the profile of the distractorinstrument is minimised.

Preferably, the formation of the spigot and socket assembly which isprovided on the distractor instrument is provided on the tibial arm. Theformation will generally be provided close to the tibial plate.

The tibial plate will generally be fastened rigidly to the tibial arm.It can be permanently connected to the tibial arm, for example by meansof appropriate fasteners, or by welding or brazing or other bondingtechniques. The tibial plate and the tibial arm can be made as a singlecomponent, for example by casting or by machining.

Preferably, the distractor mechanism comprises a pivot by which thetibial and femoral arms are connected to one another at a point betweentheir plate and control ends so that pivotal movement of the femoral armrelative to the tibial arm to reduce the distance between their controlends causes the distance between their plate ends to increase. Theportions of the tibial and femoral arms between the pivot and thecontrol ends can be gripped by a user as handles, to apply a distractingforce to the tibia and femur.

The arms can cross one another at the pivot in the manner of a pair ofscissors. However, it will often be preferred that arms do not cross,with at least one of them being cranked between the pivot and each ofits plate and control ends. Preferably, the arm which is cranked in thisway has its respective plate fastened to it in such a way that it canrotate relative to the arm so as to minimize differences in the forcethat is applied across the medial and lateral compartments of the joint.Preferably, the other arm is approximately straight, at least betweenthe pivot and the plate end. Preferably, the plate which is provided onthe straight arm is fastened rigidly to the arm so that it does notrotate.

The location of the pivot relative to the plate and control ends of thearms will be selected to provide an appropriate mechanical advantageduring operation of the distractor instrument to apply force between thefemur and tibia. It will often be preferred for the pivot to be locatedclose to the midpoint of the tibial arm so that the mechanical advantageis close to 1:1, and so that the force that is applied to the instrumentat the control ends of the arms is similar to the force that is appliedby the instrument to the patient's joint at the plate ends of the arms.

Preferably, the distractor instrument includes a device for locking thetibial and femoral arms against the relative movement between them whichinvolves reduction of the distance between their plate ends. The lockingdevice can facilitate use of the distractor instrument to provide amounting point for the ancillary instrument, providing a point ofreference relative to the resected tibia. For example, the lockingdevice might include a ratchet mechanism. A suitable ratchet mechanismmight include a toothed rack which fastened to one of the plate arms, isengaged by a projection on the other arm. Another suitable lockingdevice might include a threaded shaft and a nut which engages the shaft,as used in the device which is disclosed in U.S. Pat. No. 5,649,929.Subject matter disclosed in that document is incorporated in thespecification of this application by this reference. The distractorinstrument can also include a biasing device which acts on the tibialand femoral arms so as to reduce the distance between their plate ends.

Preferably, the femoral plate is able to rotate relative to the femoralarm so that differences in the forces that are applied across the medialand lateral compartments of the joint can be minimised. This can allow asurgeon to assess alignment of the planes on which the femur and thetibia are resected when the medial and lateral ligaments are placedunder tension, and to make such corrections as might be appropriate, forexample by further resection or by selective ligament release. The useof the distractor instrument of the invention to place the joint undertension has the advantage that the patella can remain in or close to itsanatomic position and therefore does not affect significantly therelative tensions in the medial and lateral ligaments.

In a preferred construction, the femoral plate can be fastened to thefemoral arm by means of a pin which extends from the femoral armapproximately along the anterior-posterior axis, and in which thefemoral plate can rotate about the pin. The femoral plate can have apair of recesses formed in it in which the femoral condyles can belocated, particularly when movement of the femoral plate relative to thetibial plate by the action of the distractor mechanism is restricted tomovement perpendicular to the plane of the resected tibia and does notinclude any component of movement along the medial-lateral axis. Suchrecesses can help to retain the femoral plate in contact with thecondyles, possibly during flexion of the joint. It is an advantage ofthe instrument of the invention that it can be used to apply forceacross a knee joint during flexion, enabling procedures to be performedon the joint in both extension and flexion. For example, changes in thespacing between the tibia and the femur during flexion can be monitored.

The instrument assembly of the invention can be made from materialswhich are known for use in surgical instruments. Suitable metals includecertain stainless steels. It can be preferred for the weight of theplate arms at least to be minimised. With this in mind, it can bepreferred for one or both of the plate arms to be made from a polymericmaterial. Polymeric materials that are appropriate for use in themanufacture of surgical instruments are known. They include for examplefiber reinforced resin materials, such as carbon fiber epoxy resins.

The components of the assembly of the invention should be constructed sothat they can be disassembled easily for cleaning, at least if they areintended for multiple use. Design features which meet this requirementare known from other instruments.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which:

FIG. 1 is a front view of a femur and a tibia, with a distractorinstrument according to the invention located between the femur and thetibia.

FIG. 2 is an enlarged view of the control ends of the arms of thedistractor instrument which is shown in FIG. 1.

FIG. 3 is a side view (along the medial lateral axis) of a device fordetermining the appropriate height for the anterior femur cut relativeto the tibial arm of the distractor instrument.

FIG. 4 is an isometric view of a device for measuring the gap betweenthe tibia and the femur when the joint is in flexion.

FIG. 5 is an isometric view of a device for locating a distal cuttingguide on the femur relative to the tibial arm of the distractorinstrument, according to the desired gap between the tibia and the femurwhen the joint is in extension.

Referring to the drawings, FIG. 1 shows the distractor instrument 2 ofan instrument assembly according to the invention, which comprises atibial arm 4 and a femoral arm 6. The tibial arm includes a plate end 8and a control end 10. The femoral arm includes a plate end 12 and acontrol end 14. The arms are connected to one another at a fulcrum,provided by a pivot pin 16. The pin extends through aligned holes in thearms (not shown) in an arrangement similar to what might be found in forexample a hinge.

As can be seen in FIG. 1, the tibial arm is essentially straight, andthe femoral arm is cranked towards the control end.

A tibial plate 18 is provided at the plate end of a tibial arm 8. Afemoral plate 20 is provided at the plate end of a femoral arm 10.

The connection between the arms at the fulcrum is such that movement ofthe femoral arm relative to the tibial arm so as to reduce the distancebetween the control ends of the arms causes the distance between theplate ends to increase. Similarly, movement of the femoral arm relativeto the tibial so as to increase the distance between the control ends ofthe arms causes the distance between the plate ends to decrease.

The plates are arranged on their respective arms such that the platescan be inserted into the space between the femur 102 and the tibia 104through an anterior incision, and so that the arms extend from theincision in a direction which is generally laterally of the joint. Thearms need not extend exactly parallel to the medial lateral axis. Forexample, the angle in the plane of the medial lateral axis between theaxis and the direction in which the arms extend (measured at the plateend of the tibial arm) might be as much as 30° or more, for exampleabout 40°. However, by arranging the arms so that they extend generallyin this direction, the distractor instrument can be left with the plateswithin the joint space, and the arms do not interfere significantly withthe performance of subsequent stages of the procedure. It can be amatter of surgeon choice as to whether the arms extend medially orlaterally.

A ratchet stay 22 is provided at the control end 10 of the tibial arm 4.The stay is pivotally connected to the tibial arm so that it can beswung between an operative position (as shown in FIG. 1) in which itextends between the tibial arm and the femoral arm so that it engagesthe end of the femoral arm 6, and a disengaged position. The pivotingconnection between the stay and the tibial arm is bistable biased,towards the control end of the femoral arm when in or close to itsoperative position, and away from the control end of the femoral armwhen disengaged from the femoral arm. This can be achieved by means of acam at the root of the stay and a sprung cam follower (not shown) withinthe tibial arm. FIG. 2 shows the ratchet stay in greater detail. Thestay 30 has a toothed rack on the surface which faces towards the pivotpin 16. The control end 14 of the femoral arm 6 is formed as a sharpenedblade which can fit between the teeth of the rack. A leaf spring 32 isprovided on the femoral arm towards its control end, acting between itand the tibial arm to urge them apart. Movement of the control ends ofthe arms away from one another is restricted by engagement of the bladeon the control end of the femoral arm on the ratchet stay.

The distractor instrument 2 can be used to distract a patient's kneejoint. This can be achieved by inserting the tibial and femoral plates8, 12 into the space between the resected tibia and the femur while theplate ends of the arms are close together and the control ends of thearms are spaced apart. The joint can be distracted by applying force tothe arms to close the space between their control ends, against theforce exerted between the control ends of the arms by the spring. Thedisplacement of the arms is then locked by means of the ratchet stay 22.

The tibial plate 18 is integral with the tibial arm at its plate end.The integral connection between the plate and the arm means that theplate cannot be moved relative to the arm. This can be achieved bymachining the plate and the arm from a single piece of material. It canalso be achieved by fastening separately formed plate and arm to oneanother, for example by means of appropriate clips or fasteners, or bytechniques such as welding.

The tibial plate has a planar inferior surface 24 which can fit on thesurface of the resected tibia. The tibial plate can have formations (notshown) on its inferior surface to reduce the likelihood of unwantedrelative lateral movement between the plate and the resected surface ofthe tibia. For example, the plate can be provided with pin-like orflange-like projections which are sharpened so that they can penetratethe surface of the tibia.

The tibial arm 4 has a slot 26 formed in it towards its plate end.

The femoral plate 20 is fastened to the femoral arm by means of a pin 28which passes through a hole in the femoral arm and into a bore in thefemoral plate. The plate 20 is able to rotate about the pin 28 relativeto the femoral arm 6.

The femoral plate 20 has a pair of concave recesses formed in itssuperior surface, in which the condyles can be received.

FIG. 3 shows an instrument 50 which can be used to locate the plane ofthe anterior cut of the femur 202 relative to the plane of the resectedtibia. This procedure is carried out with the knee in flexion with anangle between the femoral and tibial axes of about 90°. The instrumentcomprises an intramedullary rod 52 and a cutting block 56 which canslide on the rod along the anterior-posterior axis. The instrumentincludes a stylus 58 which is fastened to the top of the cutting block.The stylus has a slot in it, to enable it to slide on a fastening pin.The pin is sprung loaded and the resulting frictional forces between thestylus and the plate restrict relative movement between the stylus andthe block (as in other knee instruments such as that sold by DePuyInternational Limited under the trade mark LCS Completion).

The anterior cutting block has a saw cut slot 60 and fixation holes 62(which can receive bone fastening pins) provided in it. The cuttingblock 56 is able to slide along the anterior-posterior axis. Theappropriate position of the cutting block along the anterior-posterioraxis is determined using the stylus, by positioning the tip of thestylus so that it contacts the anterior cortex. This involve moving thestylus relative to the cutting block so that its tip extends beyond theend of the condyle bearing surface. It then involves moving the cuttingblock posteriorly until the tip of the stylus contacts the femoralanterior cortex. The relationship between the dimensions of the stylusand the cutting block are then such that the plane defined by the sawcut slot 60 is appropriate to enable a femoral component to be fitted tothe prepared femur.

FIG. 4 shows an ancillary instrument 70 which can be used to orientatethe cutting anterior cutting block and to measure measuring the gapbetween the tibia 204 and the femur 202 when the knee joint is inflexion with an angle between the femoral and tibial axes of about 90°.The instrument comprises a shaft 72 and a bracket 74 having alignedopenings in which the shaft can be received in a sliding fit. Thebracket has a plate 76 extending from it. The plate is dimensioned tofit into the slot 26 in the tibial arm. The shaft is cranked towards itsupper end so that the upper end 78 of the shaft is located in front ofthe femur, generally in line with the femoral axis.

An adjustable arm 80 has an elongate slot 82 formed in it. The arm isconnected to the shaft 72 at its upper end 78 by means of a threadedscrew which passes through the slot in the arm. The cutting block 56 canbe connected to the arm 80.

In use, the anterior-posterior position of the cutting block 56 isdetermined as described above with reference to FIG. 3. The cuttingblock is then connected to the adjustable arm 80 while the distractorinstrument is located between the femur and the tibia, and a force isapplied to the joint as described above, and while the plate 76 isinserted in the slot 26 in the tibial arm 4.

When the anterior cutting block 56 and the arm 80 are connected to oneanother, a scale on the shaft 72, in a window in the bracket 74,provides an indication of the distance between the tibia 204 and thefemur 202. This can be used in the selection of the size of thecomponent of the knee joint prosthesis which is to be fitted to thepatient's femur in the joint replacement procedure.

FIG. 5 shows an ancillary instrument 90 for locating a distal cuttingblock on the femur according to the desired spacing between the resectedtibia and resected femur (as determined using the flexion gap measuringinstrument 70 described above with reference to FIG. 4) when the kneejoint is extension, that is with the angle between the femoral andtibial axes being about 180°. The instrument comprises a shaft 92 and abracket 94 having aligned openings in which the shaft can be received ina sliding fit. The bracket has a plate 96 extending from it. The plateis dimensioned to fit into the slot 26 in the tibial arm.

The shaft 92 has a transverse link 98 at its upper end, with a bore 100extending through it at the end which is remote from the shaft. Thedimensions of the transverse link are such that the axis defined by thebore is directed approximately towards the notch between the condyleswhen the plate 96 is inserted in the slot 26 in the tibial arm.

The instrument 90 includes an indicator block 102 which can be used todefine the locations of fixation pins for a distal cutting block. Theinstrument enables the position of the distal cutting block to bedetermined along the mechanical axis of the femur. The mechanical axisis determined in a separate procedure as is known conventionally in kneesurgery (for example, as with the instrument set sold by DePuyInternational Limited under the trade mark LCS Completion), and definedusing a pair of pins 104. The indicator block 102 has a locator slot 106formed in it through which the pins 104 can extend so that the block canslide relative to the femur on the pins, along the mechanical axis.

The indicator block includes a transverse shaft 106 which extendsgenerally perpendicular to the mechanical axis of the femur, into thebore 100 in the transverse link 98. In this way, the position of theindicator block 102 relative to the femur can be controlled by movementof the shaft 92 in the bracket 94. The measurement obtained using theflexion gap measuring instrument 70 can be used to identify theappropriate position of the shaft in the bracket, and therefore of theindicator block relative to the femoral condyles. When the indicatorblock has been positioned appropriately along the mechanical axis of thefemur, the position is marked using locator pins which are implanted inthe femur through positioning holes 108. These locator pins can be usedsubsequently to locate a distal cutting block which can be of a knowndesign with locator holes in which the locator pins can be received anda saw guide slot. The indicator block and the distractor instrument areremoved from the joint space once the distal cutting block has beenlocated on the femur. Removal of the distractor instrument involvesreleasing the control end of the femoral arm from the ratchet stay.

The arrangement of the femoral and tibial arms extending generally alongthe medial-lateral axis allows the steps of measuring the gap betweenthe resected tibia and the femur in flexion, and of locating the planefor resecting the femur, to be carried out while the distractorinstrument of the assembly of the invention is in place with the femoraland tibial plates in the joint space placing the joint under tension.

The invention claimed is:
 1. A distractor instrument for use in asurgical procedure involving a knee joint having a tibia and a femurdefining a medial-lateral axis and an anterior-posterior axis, the femurhaving a medial condyle and a lateral condyle, and the tibia having asuperior face, comprising: a tibial plate; a femoral plate; a tibial armhaving a tibial plate end having an anterior edge, a tibial control endand opposite sides extending between the tibial plate end and tibialcontrol end, wherein the tibial plate is fastened to the tibial plateend of the tibial arm at the anterior edge, and wherein, when the tibialplate is capable of engaging with the tibia such that, the tibial plateextends posteriorly from the anterior edge in a direction generallyparallel to the anterior-posterior axis and the tibial arm extends fromthe tibial plate in a direction generally parallel to the medial-lateralaxis; a femoral arm having a femoral plate end, a femoral control endand opposite sides extending between the femoral plate end and thefemoral control end, wherein the femoral plate is fastened to thefemoral plate end, and wherein, when the femoral plate is capable ofengaging with the femur such that, the femoral plate extends posteriorlyfrom the femoral plate end in a direction generally parallel to theanterior-posterior axis and the femoral arm extends from the femoralplate in a direction generally aligned with the tibial arm; wherein thetibial arm and the femoral arm are pivotably connected to one another ata point between their respective plate and control ends so that pivotalmovement of the femoral arm relative to the tibial arm reduces thedistance between their respective control ends and increases thedistance between the tibial plate and the femoral plate; wherein an axisthrough the pivot point extends through both opposite sides of both thetibial arm and the femoral arm; and wherein at least one of the platesis offset from the corresponding arm so that the entire plate is offsetfrom both opposite sides of the arm.
 2. The instrument of claim 1,further comprising a device configured to lock the tibial arm and thefemoral arm against relative movement.
 3. The instrument of claim 1,further comprising a biasing device that acts on the tibial and femoralarms so as to reduce the distance between the tibial plate end and thefemoral plate end.
 4. The instrument of claim 1, wherein the femoralplate is fastened to the femoral arm by means of a pin that extends fromthe femoral arm along the anterior-posterior axis, and wherein thefemoral plate is configured to rotate about the pin.
 5. The instrumentof claim 1, further comprising an ancillary instrument configured to bemounted on the distractor instrument using complimentary formations of aspigot and socket assembly on one of the ancillary and distractorinstruments.
 6. The instrument of claim 5, wherein the spigot is in theform of a plate and the socket is in the form of a slot in which theplate is configured to be received.
 7. The instrument of claim 5,wherein the formation of the spigot and socket assembly provided on thedistractor instrument is provided on the tibial arm.
 8. The instrumentof claim 5, wherein the ancillary instrument is selected from: a probefor contacting the femur to measure the gap between the tibia and thefemur, a cutting guide that has a slot formed therein for receiving theblade of a saw or other cutting device, and a pin guide having at leastone hole extending therethrough to define the location for a hole thatis to be prepared in the femur.
 9. The instrument of claim 1, whereinthe tibial arm and the femoral arm are pivotably connected at a pivotpin extending generally along a line parallel to the anterior-posterioraxis.
 10. The instrument of claim 1, wherein the tibial plate isintegrally connected to the tibial plate end.
 11. The instrument ofclaim 1, wherein the tibial arm and the femoral arm are pivotallyconnected at about the midpoint of the tibial arm.
 12. The instrument ofclaim 1, wherein the tibial arm and the femoral arm do not cross oneanother.
 13. The instrument of claim 1, further comprising a ratchetstay attached to the tibial control end of the tibial arm, the ratchetstay having teeth formed on one side of the ratchet stay, and whereinthe femoral control end contacts the teeth of the ratchet stay.
 14. Theinstrument of claim 1, wherein the femoral plate has a length asmeasured along the medial-lateral axis, and the femoral plate ispivotally coupled to the femoral plate end at a point approximately themidway point of its length.
 15. The instrument of claim 1, wherein thefemoral plate extends posteriorly from the point at which the femoralplate is pivotally connected to the femoral plate end.
 16. Theinstrument of claim 1, wherein the tibial plate has a planar inferiorface sized to engage the superior face of the tibia, and wherein, whenthe planar inferior face of the tibial plate is engaged with thesuperior face of the tibia, the tibial plate extends posteriorly fromthe anterior edge in a direction generally parallel to theanterior-posterior axis and the tibial arm extends from the tibial platein a direction generally parallel to the medial-lateral axis.
 17. Theinstrument of claim 1, wherein the femoral plate has a posterior face, afirst concave recess size and shaped to accept the medial condyle, andsecond concave recess size and shaped to accept the lateral condyle, thesecond concave recess spaced from the first concave recess along themedial-lateral axis, and wherein, when the first concave recess of thefemoral plate is engaged with the medial condyle and the second concaverecess of the femoral plate is engaged with the lateral condyle, thefemoral plate extends posteriorly from the femoral plate end in adirection generally parallel to the anterior-posterior axis and thefemoral arm extends from the femoral plate in a direction generallyaligned with the tibial arm.
 18. The instrument of claim 1 wherein thefemoral plate is offset from the femoral arm so that the entire femoralplate is offset from both opposite sides of the femoral arm.